1. Field of the Invention
This invention relates to the manufacture of elongated members of thermoplastic material.
2. Summary of the Invention
An object of the invention is to produce such elongated members which simulate natural bamboo in appearance and thus have axially-spaced node-like projections of irregular shape on a stem.
According to one aspect of the invention we provide a method of producing an elongated member of thermoplastic material simulating a length of natural bamboo and comprising continuously extruding said material through a die while in a hot and fluent state into a region which permits increase in external transverse dimension of the extrudate, cooling the material downstream of the die, transporting the material away from the die and intermittently slowing the speed of transport relative to the speed of extrusion at predetermined lengths of extrusion thus to produce regions of greater external cross sectional dimensions of the extruded member to provide axially spaced node-like projections of irregular shape on the member.
An important feature of our invention is that each node produced by the method described above is slightly different to other nodes so that a length of the extrudate has nodes which are different to each other as is the case with natural bamboo so that the member simulates well the appearance of natural bamboo.
Another important feature of our invention is that we can produce both axial and circumferential irregularities in the surface of the nodes as a result of rapid cooling the surface of the extrudate a short distance downstream of the die exit by means of a water jet directed on to the surface of extrudate a short distance downstream of the die exit followed by a further rapid cooling of the extrudate for example in a water bath downstream of the jet.
It is thought that the rapid chilling action of the water jet together with the unstable wave front of the water set up on the extrudate leads to said irregulatities in the surface. We have found that the positioning of the water jet and the rate of flow are critical and the best position and flow rate must be found by trial and error with the particular material being extruded and the rate and other parameters of extrusion concerned to give the best simulation of natural bamboo.
Since the extrudate must enter the water bath through a flexible sealing member contact between the member and the extrudate can contribute to the formation of irregularities in the surface of the nodes.
If desired, instead of a water bath a plurality of water jets may be provided.
The member is preferably tubular in which case the projections are accompanied by an increase in the cross-sectional bore of the tubular member.
The thermoplastic material may be a rigid thermoplastic material such as polyvinyl chloride.
If desired the extrudate may be reinforced by a tube of metal or other relatively rigid material in which case less rigid thermoplastic material, such as polythene, may be used.
If there is no variation in the relative speed of extrusion and transport between said predetermined lengths we can obtain a member of generally constant cross-section between the projections thus simulating the appearance of natural bamboo where the anti-node portions are generally of constant cross section. Preferably, the extrusion is maintained at a substantially constant speed and the transport is intermittently stopped to produce the node like projections. The thermoplastic material continues to flow out of the die while the transport is stopped thus producing the hereinbefore described projections which simulate the nodes of natural bamboo. Normally the speed of transport between the stops is such as to give a generally constant cross section to the part of the member which simulates the anti-nodes of natural bamboo.
An apparatus for producing an elongated member of thermoplastic material simulating a length of natural bamboo may comprise means for continuously extruding said material through a die while in a hot and fluent state into a region which permits increase in external transverse dimension of the extrudate, means for cooling the extruded material downstream of the die, means downstream of the cooling means for gripping the cool material and means for driving the gripping means to transport the material away from the die at a cyclically variable speed.
Preferably, the gripping means comprises a pair of caterpillar devices and these are driven intermittently through a one-way clutch; the one-way clutch may be driven by a fluid cylinder and the stoke of the cylinder may be varied to vary the length of the member between adjacent projections. Alternatively the caterpillar devices may be driven from a continuously running motor by way of a clutch which may be controlled by a fluid cylinder.
The cooling means may comprise a water jet immediately downstream of the die followed by a further cooling means.
The further cooling means may comprise a water bath.
The extrudate may enter the water bath through a sealing member which contacts the surface of the extrudate and contributes to the formation of irregularities in the surface of projections formed on the extruded material as a result of slowing of the speed of transport relative to the speed of extrusion by said means for driving the gripping means at a cyclically variable speed.